1. Field of the Invention
The present invention relates to an apparatus for producing monocrystals utilized as medical scintillators or the like. More particularly, the present invention relates to a production apparatus that is utilized in producing monocrystals through the pulling-down method, especially through the rapid melt-growth method that is called the micro-PD method, and to a vacuum container provided in the production apparatus.
2. Related Background Art
So-called scintillator crystals having a characteristic of converting a radiant ray into light include, for example, oxide-system crystals such as LSO, GSO, and BGO and fluoride-system crystals such as GLF and GYF are known. When utilized as a medical scintillator, the crystals are required to have high fluorescence intensity and a short fluorescence lifetime. To date, it has been believed that, when the required characteristics are taken into consideration, oxide-system crystals are promising. The oxide-system crystals are obtained generally through the CZ method (so-called the Chokralsky method). However, the CZ method requires a process in which the temperature rises to 2000° C. or higher during the production; therefore, it costs much to build the production apparatus, and the time necessary for crystal growth is long. Accordingly, production costs are raised; therefore, it can be considered that the crystals obtained through the CZ method may hardly be suitable for mass production.
In this situation, the pulling-down method is known as a method in which, despite the small diameter of a crystal to be obtained, the time necessary for crystal growth is short, and, compared with the CZ method, the crystal is obtained at low costs and superior in crystallinity (Japanese Patent Application Laid-Open No. 2001-080999 (Japanese Patent No. 3521070) and Japanese Patent Application Laid-Open No. H11-278994). In the method, a monocrystal made of a melt material is obtained by providing in the middle portion of a melting pot for retaining a melt material a narrow hole ranging from the inside to the outside of the melting pot, making a seed crystal (referred to as a seed, hereinafter) contact the melt material that leaks through the narrow hole, and pulling down the seed concurrently with the crystallization of the melt material.
In the case where, through the pulling-down method, good crystallinity should be obtained, it is required to maintain the flattening of a solid-liquid interface. However, in the pulling-down method utilizing a melt material that leaks by gravity through a narrow hole, for example, there may be a problem in that, depending on the viscosity of the melt material, the melt portion is broken through its own weight, or the diameter of a monocrystal is rendered large due to so-called liquid dripping. Therefore, it is considered impractical to relax too much the temperature gradient at the solid-liquid interface as a crystal growth portion, or in the vicinity of the solid-liquid interface. In other words, in accordance with conditions such as the viscosity of the melt material, a certain degree of crystal-growth speed or seed-pulling-down speed is required to be maintained. It is considered that it is liable to be difficult to maintain the flattening of the solid-liquid interface under these conditions.